Targets and processes for fabricating same

ABSTRACT

In particular embodiments, the present disclosure provides targets including a metal layer and defining a hollow inner surface. The hollow inner surface has an internal apex. The distance between at least two opposing points of the internal apex is less than about 15 μm. In particular examples, the distance is less than about 1 μm. Particular implementations of the targets are free standing. The targets have a number of disclosed shaped, including cones, pyramids, hemispheres, and capped structures. The present disclosure also provides arrays of such targets. Also provided are methods of forming targets, such as the disclosed targets, using lithographic techniques, such as photolithographic techniques. In particular examples, a target mold is formed from a silicon wafer and then one or more sides of the mold are coated with a target material, such as one or more metals.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/066,479, filed Mar. 11, 2008, which is the U.S. National Stage ofInternational Application No. PCT/US2006/035267, filed Sep. 12, 2006,which was published in English under PCT Article 21(2), which in turnclaims the benefit of U.S. Provisional Application No. 60/716,540, filedSep. 12, 2005; and U.S. Provisional Application No. 60/776,268, filedFeb. 24, 2006. Each of these applications is incorporated by referenceherein in its entirety.

GOVERNMENT SUPPORT

This invention was made with government support under grantDE-FC52-01NV14050 awarded by the United States Department of Energy. Thegovernment has certain rights in the invention.

TECHNICAL FIELD

The present disclosure relates to targets and their methods offabrication. In particular examples, the present disclosure providemethods of fabricating metal targets useable as laser targets inhigh-energy laser-physics.

BACKGROUND

Metal covered targets are typically used in high energy physicsapplications. For examples, such targets may be shot with a laser inorder to generate plasmas or high energy radiation. Such targets may beused in applications such as inertial confinement fusion.

Targets for lasers used to produce plasma and radiation typically havedisadvantages. For example, conventional targets are often produced bymicro-machining processes that typically produce targets having a tipsharpness, or apex dimensions, of 25 μm or larger. For example, acurrent process involves micro-machining a mandrel, electroplating themandrel with a desired metal, and then etching away the mandrel. Otherprocesses involve depositing a metal layer on a plastic mold and thenmelting away the plastic mold. The tips of targets produced by suchprocesses can be significantly larger than the wavelength of the laserlight that will be used with the target and therefore may not produceoptimal energy. Similarly, the apexes, or tips, of the targets can belarger than the focal size (or spot size) of the laser, which canminimize any enhancements that might otherwise be conferred by thetarget shape.

In addition, such targets are typically manufactured individually andthus can be comparatively expensive. The expense of the targets maylimit the number of targets available for use, thus potentially limitinghow the targets can be used. For example, a limited number of targetsavailable for a series of experiments may limit the quality or quantityof data obtained during the experiments.

The amount of material available on such targets or irregularities inthe target surface may interfere with full characterization of theproduced plasma. Insufficient target material may also interfere withoptimal energy production.

Some prior experiments have used metal coated silicon targets. However,the silicon included in such targets typically interferes with energyfocusing and radiation enhancement.

While hemispherical laser targets have been tested, such targetstypically suffer from disadvantages in addition to those noted above.For example, irregularities in the surface of the target, or variationsin the targets resulting from their method of manufacture, may make itdifficult to properly position the target and position other objectswith respect to the target.

SUMMARY

The present disclosure provides hollow targets having a metal layer andany combination of straight or curved surfaces and an internal apex ofless than about 15 μm, such as less than about 10 μm. In specificexamples, the internal apex is less than about 1 μm. In particularembodiments, the metal targets are free-standing. In furtherembodiments, the targets are arranged in arrays. Some disclosed targetsare surrounded by a protective frame or a structure that aids inmanipulating the targets.

The present disclosure also provides methods of lithographicallyfabricating targets, including the above-mentioned targets having aninternal apex of less than about 15 μm. A particular method of thepresent disclosure provides for forming a free-standing pyramid-shapedtarget. An aperture is formed in a front side of a masked silicon wafer.The front side of the silicon wafer is etched along diagonal planes toform a pyramidal void. A layer of a target material, such as a metal, isthen formed on the surface of the pyramidal void. In particularexamples, the target material is selected from Au, Pt, Cr, Cu, Pd, Ta,Ag, Ti, W, silicon nitride, and polysilicon. In some instances, anadhesion layer is formed prior to forming the target material layer. Aback side of the silicon wafer is etched to expose a surface of thetarget material.

In a particular implementation, a thin layer, such as less than 2 μm, ofa first target material is formed on the pyramidal void and then asecond target material is deposited on the target, such as on the backside of the first target material once the back surface has been exposedas described above.

In another embodiment, the present disclosure provides a method offorming a conical target. A thin film, such as about 1000 Å of SiO₂ isdeposited on a silicon wafer, such as double polished silicon. A thinlayer of silicon nitride, such as about 1000 Å, is then deposited on theSiO₂ layer. Standard photolithography techniques are used to form anopening, such as an opening of at least about 400 μm, on one surface ofthe wafer. The surface is etched to form a conical void under theopening.

An opening, such as an opening of at least about 400 μm formed in theopposing surface of the wafer. An etch is performed in the opening inthe opposing surface of the wafer until all sides converge at the tip ofthe pyramid formed in the surface of the wafer, making an outerpyramid-shaped silicon mold. In certain examples, standard oxidesharpening techniques are used to create a sharper tip. A metal coating,such as a coating of at least about 1.5 μm, is deposited on thepyramidal void to create a pyramidal metal structure. In particularexamples, a second metal is deposited on top of the first metal. Thesilicon mold is then etched away from the back surface of the siliconwafer to create a hollow free-standing metal pyramid or hollow pointedmetal target. In specific examples, the target has an internal apex ofless than 1 μm. Other shapes may be formed using the disclosed methods,such as cones.

Further embodiments provide methods of forming hemispherical targets. Abackside of a silicon wafer is coated with photoresist. At least oneaperture is formed in the backside and the backside is etched. Thebackside photoresist is then removed and the front side of the wafer iscoated with photoresist. Apertures are then formed in the front side ofthe wafer using standard techniques and the remaining photoresist formedinto domes using acetone reflow. The front side is then etched to removeboth silicon and photoresist and transfer the dome shaped to the siliconwafer. A metal layer is then formed on the front side of the wafer.

The presently disclosed hemispherical targets can provide advantagesover previous targets. For example, the disclosed fabrication techniquescan allow the surface, such as the lens diameter and radius ofcurvature, of the target to be controlled and tailored for a particularapplication. Knowing the curvature and other dimensions of the targetcan aid both in positioning the target and positioning other objectswith respect to the target.

Yet further embodiments of the present disclosure provide capped ortopped targets, such as conical or pyramidal targets having a topextending horizontally from the apex of the conical or pyramidal target.The tops can have various shapes, including square, circular,rectangular, parallelogram, hexagonal, pentagonal, elliptical, crossshaped, or an arbitrary shape. In some implementations, the cap is madeof a single metal. In further embodiments, the cap includes multiplemetals. The metals can have the same or different shapes or thicknesses.In one particular example, the cap includes concentric circles ofvarious metals. In a further example, the cap includes a first metallayer covered with a polka dot pattern of a second metal. In someconfigurations the cap is hollow while in other configurations the capis solid.

In some implementations, the target is attached to a base piece by asupport Structure. In a particular example, the support structure is ahorizontal extension of a cap portion. In more particular examples,openings are formed in the support arm. Attachment to a base piece canallow for easier manipulation of the target.

Capped targets can provide a number of advantages, including a larger ormore regular surface area, which may allow resulting plasma or otheremissions to be characterized or allow more energy to be produced. Theability to use different metals, in the base and cap or either the baseor cap, can aid in tailoring the type of energy emitted from the target.

The present disclosure provides methods for fabricating capped targets.According to a disclosed method, a film of mask material, such assilicon dioxide, is deposited on both sides of a silicon wafer. Anaperture is etched into the backside of the wafer. Standardphotolithography techniques are used to create a target opening of asuitable size and shape for the cap of the target. One or more metallayers, or other target material, are then deposited in the targetopening. Extraneous metal can be removed, such as by using standardlift-off techniques.

Apertures flanking the metal layers are created using standardphotolithography techniques. The etch process is stopped short such thata target base, such as a conical or pyramidal target, is left supportingthe metal cap. A layer of metal, or mask material, is then deposited onthe front side of the wafer and the rear side of the wafer etched toremove silicon from the inside of the target base. If desired, the frontside mask material can then be removed.

The disclosed methods can allow targets to be fabricated more cheaply,easily, consistently, or controllably than prior methods. Particularmethods can allow targets to be mass fabricated. In a particularexample, arrays of targets can be fabricated. The availability ofgreater numbers of targets, or greater varieties of targets, can allowthe targets to be used in new applications, as well as potentiallyincreasing the quality or quantity of data obtainable from experimentsusing the targets.

The sharp, often submicron, dimensions of the inside apex or tip of theconical, pyramid, hemispherical, or otherwise hollow metal targets cansignificantly enhance the brightness for emitted radiation, such asx-rays, and the amount of particles produced. When the laser light getsfocused, after the focusing optic, it can be further focused by theinner surface of the disclosed targets. In some cases, such focusingproduces more energy or may allow pointing requirements for the laser tobe relaxed.

There are additional features and advantages of the subject matterdescribed herein. They will become apparent as this specificationproceeds.

In this regard, it is to be understood that this is a brief summary ofvarying aspects of the subject matter described herein. The variousfeatures described in this section and below for various embodiments maybe used in combination or separately. Any particular embodiment need notprovide all features noted above, nor solve all problems or address allissues in the prior art noted above.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments are shown and described in connection with thefollowing drawings in which:

FIGS. 1A through 1L are cross sectional diagrams illustrating a processfor forming targets according to variations of a first aspect of thepresent disclosure.

FIGS. 2A through 2H are cross sectional diagrams depicting a process forforming targets according to variations of a second aspect of thepresent disclosure.

FIG. 3 is an illustrative mask layout for a photomask that may be usedin etching the front side of the wafer in the process of FIGS. 2Athrough 2H.

FIG. 4 is an illustrative mask layout for a photomask that may be usedin etching the back side of the wafer in the process of FIGS. 2A through2H.

FIGS. 5A through 5D are schematic representations of an etching progressthat can be used to create a pyramidal void.

FIGS. 6A through 6F are cross sectional diagrams illustrating a processfor forming targets according to another aspect of the presentdisclosure.

FIG. 7 is an illustrative mask layout for a photomask that may be usedin etching the back side of the wafer in the process of FIGS. 6A through6F.

FIGS. 8A through 8I are cross sectional diagrams illustrating a processfor forming hemispherical targets according to the present disclosure.

FIGS. 9A and 9B are, respectively, front side and backside masks thatcan be used in the process of FIGS. 8A through 8I.

FIG. 10 is an electron micrograph of a flat topped target according tothe present disclosure.

FIGS. 11A through 11G are cross sectional diagrams illustrating aprocess for forming the flat topped target of FIG. 10.

FIGS. 12A through 12D illustrate various types of caps or tops that maybe used in capped targets according to the present disclosure.

FIG. 13 is a top plan view of an embodiment of a capped target accordingto the present disclosure that is linked by a structural connection to abase piece.

DETAILED DESCRIPTION

Unless otherwise explained, all technical and scientific terms usedherein have the same meaning as commonly understood by one of ordinaryskill in the art to which this disclosure belongs. In case of conflict,the present specification, including explanations of terms, willcontrol. The singular terms “a,” “an,” and “the” include pluralreferents unless context clearly indicates otherwise. Similarly, theword “or” is intended to include “and” unless the context clearlyindicates otherwise. The term “comprising” means “including;” hence,“comprising A or B” means including A or B, as well as A and B together.Although methods and materials similar or equivalent to those describedherein can be used in the practice or testing of the present disclosure,suitable methods and materials are described herein. The disclosedmaterials, methods, and examples are illustrative only and not intendedto be limiting.

Conical and Pyramidal Targets

Referring first to FIGS. 1A through 1L, cross sectional diagrams showthe progressive processing for forming targets according to variationsof a first aspect of the present disclosure.

Referring now to FIG. 1A, the submicron-tip pyramid-shaped target isformed on a silicon wafer 10, such as a wafer 10 having a <100> crystalorientation. In a particular example, the wafer 10 is about 525 μmthick. First, both the front side 12 and back side 14 of the wafer 10are blanket coated with about 1000 angstroms of silicon nitride 16 and18, respectively, using standard semiconductor processing techniques.The silicon nitride layer is double polished and then about 1.6 μm ofphotoresist 19, such as Shipley 3612, is deposited on the front sidesilicon nitride layer 16. In a particular example, the wafer 10 isprimed with Hexamethyldisilazane (HMDS) before applying the photoresist.The wafer 10 is then soft baked at 90° C. Although a silicon nitridemask material is described, other mask materials, such as silicondioxide, may be used if desired.

Referring now to FIG. 1B, the front side 12 of the wafer 10 is patternedusing conventional photolithography techniques, such as by exposing thewafer 10 to the desired mask pattern for a suitable period of time, suchas about 1.7 seconds. In some examples, the wafer 10 is developed usingLDD26W (available from Shipley Co.) developer and a 110° C. postbake.The silicon nitride is etched using standard semiconductor processingtechniques, such as using a RIE (reactive ion etch) dry etch for 4minutes, to clear the target window 20 and release-tab windows 22. In aparticular example, the etch rate is about 300 Å/m. In a particularexample, the RIE employs a mixture of SF₆ and O₂. Remaining photoresistcan be stripped by a suitable process, such as a standard O₂ etch. FIG.1B shows the structure resulting after these processing steps have beenperformed.

Referring now to FIG. 1C, the target window 20 and the release-tabwindows 22 are etched to a depth of just over about 400 microns to forma pyramid-shaped target depression 24 using silicon nitride layer 12 asa mask. For a square target window measuring about 500 microns on aside, a nominal etch time of about 5 hours in KOH with a 1 hourover-etch time is used. In a particular example, the etch is performedat about 90° C. The object is to etch until the crystal planes in thetarget window 20 intersect or converge. The etch process is continueduntil substantially all of <100> plane has been etched away (see FIGS.4A through 4D, particularly FIG. 4D), resulting in the intersection at apoint of the planes comprising the pyramidal surfaces of the completedetch pit in order to achieve submicron tip resolution in the targetwindow 20. With a 500 micron wide target window 20, the completed etchdepth will be about 400 microns. The angle of the pyramidal void formedwill be about 70.6° using this etch process. Slower etch rates typicallyyield smoother sidewalls. Appropriate KOH decontamination procedures areperformed as is known in the art, such as treatment with 5:1:1H₂O:H₂O₂:HCl. Organics can be stripped using suitable techniques, suchas treatment with 9:1 H₂SO₄:H₂O₂. FIG. 1C shows the structure resultingafter these processing steps have been performed.

Appropriate selection of etch conditions can be used to control theshape of the target window 20. For example, while KOH produces pyramidaltargets having a target pyramidal angle of 70.6°, adding a nonionicsurfactant to TMAH (Tetra Methyl Ammonium Hydroxide) and using {110}silicon, conical windows 20, with a cone angle of 90°, can be formed.

Referring now to FIG. 1D, the back side 14 of the wafer 10 is patternedusing conventional photolithography techniques, such as being spun withShipley 3612 or Shipley 220 photoresist and exposed and developed usinga suitable backside mask. In a particular example, the backside 14 iscoated with about 4 μm of photoresist after a HMDS prime and then thewafer is soft baked at 90° C. In a further example, the backside 14 isexposed with the mask for about 4.5 seconds, developed with LDD26Wdeveloper, and not postbaked. The silicon nitride layer 18 on the backside of wafer 10 is etched using standard silicon nitride etchingtechniques, such as the RIE process for 4 minutes, to clear the backside etch windows 26. In another example, the nitride etching process isperformed in about 2.5 minutes. FIG. 1D shows the structure resultingafter these processing steps have been performed.

Referring now to FIG. 1E, the back side 18 of the silicon wafer 10 isetched in an anisotropic dry etching process using silicon nitride layer18 as a mask. In a particular example, an STS dry etch is used (SurfaceTechnology Systems, plc. of Newport, UK). About 80-90 microns ofmaterial is removed. The etching process is stopped short of exposingthe tips of the target depressions. In a particular example, the STSetch is performed over a period of about 25 minutes. Photoresist andorganics can then be stripped, such as by using 9:1 H₂SO₄:H₂O₂. FIG. 1Eshows the structure resulting after these processing steps have beenperformed.

Next, depending on whether or not a submicron tip is needed, theprocessing shown in either FIG. 1F or 1G is performed. If a submicrontip is not desired, the processes of FIG. 1F are performed.

Referring now to FIG. 1F, an adhesion layer 28 is blanket deposited onthe front side of the wafer 10 using, for example, e-beam evaporationand thermal evaporation or sputtering. A metal such as Ti issatisfactory for use as the adhesion layer. Other materials may be useddepending on their compatibility with the metal layer that is to be usedto form the target. A thickness of about 100 angstroms has been found tobe satisfactory for the adhesion layer.

Next, a layer of a target material 30 is blanket deposited over theadhesion layer 28 using e-beam or thermal evaporation, sputtering orelectroplating techniques. This layer may be from about 8 to about 10microns in thickness, in particular examples. In furtherimplementations, the layer is at least about 8 microns thick. Athickness of 1.7 microns has been found to be satisfactory. FIG. 1Fshows the structure resulting after the adhesion layer 28 and the10-micron thick target-material layer 30 have been deposited. If a10-micron thick layer of target material is deposited, the targetmaterial may form a meniscus at the bottom of the pyramid-shaped targetdepression 24, which may cause the tip not to have submicron dimensions,although the target formed may still be useful for certain applications.

The target material may be any material that is suitable for use as atarget in the procedure that will employ the target of the presentdisclosure. Materials including metals such as Au are suitablematerials, although other metals and other known target materials may beemployed. The target material may be deposited using known methodssuitable for forming such target materials such as CVD, LPCVD,evaporation, sputtering, electroplating and other known materialsprocessing methods. Persons of ordinary skill in the art will appreciatethat, depending on the composition of the target material and the degreeof its adhesion to the substrate material, the adhesion layer 28 may notbe necessary.

If it is desired to form a submicron tip, the processing shown in FIG.1G is performed. Referring now to FIG. 1G, an adhesion layer 28 isblanket deposited on the front side of the wafer 10 using, for example,e-beam evaporation. This is the same adhesion layer that would have beenformed if the processing indicated in FIG. 1F had been performed. Athickness of about 100 angstroms has been found to be satisfactory forthis layer. Next, a layer of target material 32 is blanket depositedover the adhesion layer 28 using e-beam and thermal evaporation orsputtering techniques. However, unlike the processing illustrated withreference to FIG. 1F, the thickness of the target-material layer 32 maybe from about 1.5 microns to about 2 microns. A thickness of 1.7 micronshas been found to be satisfactory. FIG. 1G shows the structure resultingafter the adhesion layer 28 and the 1.5 to 2 micron target-materiallayer 32 have been deposited.

Referring now to FIG. 1H, the back side 18 of wafer 10 is etched using asuitable etchant (such as KOH at 90° C.) to expose the target-materiallayer 30, now a freestanding pyramid, and the wafer 10 is suitablydecontaminated. The target-material layer 30 acts as an etch stop and,depending on its composition, the adhesion layer may be removed by theetchant. In the case of Ti adhesion layer and Au target material, KOHwill perform well. Care should typically be taken to ensure that thewafer 10 is not over-etched, as this can cause the target to releasefrom the wafer 10. In a particular example, the KOH etch is performedfor about 100 minutes, such as to etch about 150 μm. Similarly, careshould be used in performing KOH decontamination steps (such as using5:1:1 H₂O:H₂O₂:HCl) to avoid release of the targets.

Referring now to FIG. 1I, a layer of target material 34 is deposited onthe back side of the wafer. This layer 34 may be about 8-10 micronsthick, in particular examples. An adhesion layer, such as a Ti adhesionlayer, may be used if desired. The processing steps referred to withrespect to FIG. 1I are not performed if the processing steps referred towith reference to FIG. 1F are performed, since a thicker layer of targetmaterial that is capable of free standing by itself is already presentin the structure.

According to one variation of the present disclosure illustrated withreference to FIG. 1J, a multiple-metal target may be formed. Accordingto this embodiment of the disclosure, instead of depositing a singlelayer 34 of target material as shown in FIG. 1I, a thin film 36 (e.g.,having a thickness in the range of between about 0.1 μm up to 10 μm,such as from 0.5 μm to 2 μm) of a target material such as a metal isfirst coated onto the exposed Ti surface 28. In particular examples,evaporation is used as the deposition method since evaporation willline-of-sight deposit the material, focusing the deposition at the apex.In further examples, the metal is coated using sputtering orelectroplating. Next, a layer 38 of Au is deposited on the back side ofthe wafer over layer 34 to a thickness of about, for example, 10microns. FIG. 1J shows the structure resulting after the Au layer 38 hasbeen deposited.

According to another variation of the present disclosure, thesubmicron-tip pyramid-shaped target of the present disclosure may beformed in a process that employs a sacrificial layer for use in formingthe target. This aspect of the present disclosure employs the process asillustrated in FIGS. 1A through 1E to the point where the back side 18of the silicon wafer 10 has been etched, stopping short of exposing thetips of the target depressions. Then, as illustrated in FIG. 1K, asacrificial layer 40 of a material such as silicon nitride, polysilicon,Mo, Ni, Pd, Pt, Cu or Ag, or other material that will withstand thesubsequent etch back step, is deposited on the front side of the waferand into the target depressions. In a particular example, thesacrificial layer is a 2 μm layer of silicon nitride.

The back side of the wafer is then further isotropically etched toexpose the lower surface of the sacrificial material serving as thefreestanding pyramid mold. In a particular example, a KOH wet etch isused and the wafer suitably decontaminated after the etching step.

FIG. 1K shows the structure resulting after the back side of thesacrificial layer has been exposed by the etching step. The sacrificiallayer can be preferentially etched away thereby leaving layer 42 as theonly freestanding metal structure.

A different metal than the desired device metal can be used to act asthe sacrificial freestanding mold and then later removed. Mo, Ni, Pd,Pt, Cu, Ag or highly doped silicon will work, as they are all imperviousto the KOH solution and may be selectively etched away later on in theprocess. Once the sacrificial mold is freestanding, the desired metalcan be deposited to the backside of the sacrificial freestanding mold.In the case of Ni as the sacrificial mold and Au as the desired devicematerial; the Ni will etch away in a standard wet etch known in the artas Piranha etch and it will not affect the gold (all metals describedabove as the sacrificial mold will etch in Piranha solution, but goldwill not). What is left will be a freestanding gold device that has evena sharper inside apex, as no apex resolution is lost because all of themetal from the inside deposition that collected at the tip of thesilicon mold will be removed.

In this process, the freestanding metal structure can be used as thetemporary freestanding mold and then later removed, allowing only themetal that was deposited to the backside remain as the freestandingdevice. This process can provide a number of advantages. The process canallow sharper inside apexes to be formed and can result in targets whichare less expensive to manufacture.

Next, as illustrated in FIG. 1L, a layer 42 of desired target materialis then deposited, such as by sputtering or evaporation. This materialmay be, for example, a layer of Au having a thickness of about 10microns, or a layer of another target material. The sacrificial layer isthen removed using an etching process that differentiates between thetarget material layer 42 and the sacrificial layer 40. FIG. 1L shows thestructure resulting after the sacrificial layer has been removed,leaving the free-standing pyramidal target.

Referring now to FIGS. 2A through 2I, another aspect of the presentdisclosure is illustrated. Referring now to FIG. 2A, a silicon wafer 80has its front side 84 and back side 86 covered with deposited siliconnitride layers 86 and 88, respectively. A layer of photoresist 90 isformed and patterned on the front side 82 of the wafer 80 to form anaperture 92. The portion of silicon nitride layer 86 in aperture 92 isetched away to expose the front side 82 of wafer 80 as shown in FIG. 2A.The photoresist layer 90 is then stripped using conventional processingtechniques.

Referring now to FIG. 2B, another layer of photoresist 94 is applied tothe front side 82 of wafer 80 and patterned to form aperture 96 withinthe region from which the front side silicon nitride layer 86 has beenremoved. FIG. 2B shows the structure resulting after these processingsteps have been performed.

Referring now to FIG. 2C, a metal layer 98 is deposited on the frontsurface of the wafer, both over photoresist layer 94 and over theexposed portion of the front side 82 of wafer 80 in aperture 96. About3,000 angstroms of gold has been found to be satisfactory for thispurpose although persons of ordinary skill in the art will appreciatethat other metals and other thicknesses will also function for thislayer. FIG. 2C shows the structure resulting after metal layer 98 hasbeen deposited.

Referring now to FIG. 2D, a standard metal liftoff procedure isperformed to remove the unnecessary metal above the photoresist layer 94as well as the photoresist layer 94, leaving the portion of gold layer96 and silicon nitride layer 84 deposited on the front surface 82 ofwafer 80. FIG. 2D shows the structure resulting after the unnecessaryportions of metal layer 98 and all of the photoresist layer 94 has beenremoved.

Referring now to FIG. 2E, a DRIE anisotropic silicon etch is performed,using a process such as the Bosch process in an STS etcher. As will beappreciated by persons of ordinary skill in the art, the Bosch processis a dry etch performed with the sidewalls passivated with a polymerfrom the dry etch chemistry, which allows deep, highly anisotropicetching (e.g., at ratios such as 150:1) into the surface of the silicon.This etch may be performed to a depth of 150 μm to 250 μm, such as toabout 200 μm, with a width of the metal layer 98 of 250 microns to 350μm, such as to about 300 μm. FIG. 2E shows the structure resulting afterthe anisotropic etching step has been performed, including the voids 100formed by removing portions of the wafer.

Referring now to FIG. 2F, the tip 102 of the target to be formed isdefined using an anisotropic wet or dry isotropic silicon etchingtechnique. The etching process will undercut the region of the siliconwafer underlying metal layer 98, causing the tops of the sidewall edgesto form a submicron featured point 102 as they meet from opposite sidesas shown in FIG. 2F. Metal layer 98 will be removed as a result of thisetching process.

Referring now to FIG. 2G, a metal layer 104 is formed over the exposedfront surface 82 of the wafer 80. In one embodiment of the disclosure,the metal layer 104 is a layer of Au formed to a thickness of about 1.7μm. Next a layer 106 of another metal, such as Ti, Cu, or Ag, is formedto a thickness from 100 nm to 8 μm. Finally, another Au layer 108 may beformed to a thickness of about 1 μm. The thicknesses of layers 104, 106,and 108 are not critical.

Referring now to FIG. 2H, a photomask layer 110 is formed over the backside 84 of wafer 80 and exposed to form aperture 112 aligned with thetip of the metal layer. An anisotropic silicon etching process isperformed to remove all of the silicon material disposed below metallayer 104 in the aperture 100. The masking layer 110 is then removed.

Referring now to FIG. 3, a typical mask 120 for use in processing thefront side 12 of the wafer 10 is shown. This mask is used in the processof FIG. 1B. This mask has a central aperture 122 for the target and fourrelease windows 124 at the periphery. The central aperture 122 is usedto form target window 20 shown in FIG. 1B. The release windows 124 allowindividual ones of the freestanding pyramid-shaped targets to beseparated from each other after the completion of processing and areused to form release-tab windows 22 shown in FIG. 1B.

Referring now to FIG. 4, a typical mask 130 for use in processing theback side 12 of the wafer 10 is shown. This mask is used in the etchingprocess described with reference to FIG. 1E. This mask has a firstaperture 132 for the target and four release windows 134 at theperiphery. The first aperture 132 is used to define support members thatwill be formed in locations 134 to form a frame for the free-standingtarget. The release windows 136 allow individual ones of thefree-standing pyramid-shaped targets to be separated from each otherafter the completion of processing and are used to form etch windowsthat will join the etched voids defined by release-tab windows 22 shownin FIG. 1B.

Referring now to FIGS. 5A through 5D, schematic representations of theetching progress to form a pyramidal void through target aperture 20 areshown. First, at FIG. 5A, an under-etch condition is shown in which aportion of the <100> plane 140 can still be seen. Next at FIGS. 5B, and5C the portion 140 of the <100> that remains becomes progressivelysmaller. FIG. 5B has utility in that by purposely extending the plane of140 more of a trough shaped target is obtained which may be of interest.Finally, FIG. 5D shows an ideal etch-progress condition in which theetch has proceeded to such an extent that the <100> plane has beenetched away, resulting in the perfect intersection at a point of theplanes comprising the pyramidal surfaces of the completed etch pit.

Referring now to FIGS. 6A through 6F, cross sectional diagrams show theprogressive processing for forming targets according to another aspectof the present disclosure.

First, as shown in FIG. 6A, the process begins with a double polished<100> silicon substrate 150. A substrate of <110> silicon will work aswell. A film 152 of SiO₂, then, optionally, a film of silicon nitride154 is formed on the substrate 150 using standard LPCVD (Low PressureChemical Vapor Deposition) techniques. A thickness of about 1,000angstroms has been found to be suitable for these layers 152 and 154,although other thicknesses could be employed. For example, analternative embodiment employs a silicon dioxide mask of at least about4 μm. FIG. 6A shows the structure resulting after these processing stepshave been performed.

Referring now to FIG. 6B, a cross-shaped opening with a circular plug inthe center (see FIG. 7) is patterned on the surface of the siliconnitride layer 154 using standard photolithography techniques. Adimension of about 1 mm on all sides has been found to work well for a 5mm×5 mm die size with a circular feature having a diameter of about 400μm. Standard etching techniques such as a nitride/oxide dry etchtechniques are then performed to clear the SiO₂ and silicon nitridelayers 152 and 154 from the large cross-shaped window 156 down to thesilicon substrate 150, leaving the circular plug 158 in the center. FIG.6B shows the structure resulting after these processing steps have beenperformed.

Referring now to FIG. 6C, a square opening 160 (having, for example, adimension of about 400 μm per side) is patterned on the backside of thesilicon wafer 150 using standard photolithography techniques. Thenstandard etching techniques, such as nitride/oxide dry etch techniques,may be performed to remove the SiO₂ and silicon nitride layers 152 and154 to clear the window. This window provides the opening when etchingback the remaining silicon mold to create a free-standing device. FIG.6C shows the structure resulting after these processing steps have beenperformed.

Referring now to FIG. 6D, an etching process such as an isotropic dryetch is performed until all sides converge at the tip making an outercone or pointed silicon mold. The isotropic etch will allow undercuttingalong all sides of the circle, eventually forcing all sides to convergeto a point 162 and forming a cone or pointed mold 164. A depth ofgreater than about 400 μm is typically used. A perfect isotropic etchwill have the same etch rate in the vertical direction as the horizontaldirection (ratio: 1:1). One suitable dry etch chemistry is XeF₂. Asuitable etcher with etch rates of up to 10 μm/min is available fromXactix, Inc., of Pittsburgh, Pa. In further embodiments, the etch isperformed using an STS etcher and eliminating the passivation step. FIG.6D shows the structure resulting after these processing steps have beenperformed.

After the cone or pointed mold is defined, standard oxide sharpeningtechniques such as oxide enhanced sharpening may be performed to createa sharper tip. In a particular example, the tip sharpening techniqueinvolves dry thermal oxidation (for example, at about 1000° C. for aboutan hour), such as to add about 1500 Å of silicon dioxide, followed by aHF etch. This process can be repeated, as desired, to achieve aparticular tip sharpness or sidewall roughness.

Referring now to FIG. 6E, a layer 166 of the desired target metal isdeposited to a desired thickness on top of the silicon cone or pointedmold 164. The thickness of metal layer 166 is typically greater thanabout 1.5 μm if a freestanding metal target is desired. In particularexamples, 10 μm of a metal such as gold or the other materials disclosedherein are deposited. FIG. 6E shows the structure resulting after theseprocessing steps have been performed.

Referring now to FIG. 6F an etch back process is performed on the backside of the substrate 150 to eliminate the silicon mold 164 to create afree-standing metal structure 168. This can be done using a wet or dryetch. The SiO₂ layer is used as the preferred masking layer for the XeF₂dry etch and the Silicon Nitride layer is the preferred masking layerwhen performing the wet etch. If a wet etch is not used for the etchback step, then silicon nitride layer 164 is not necessary and may beskipped during the steps shown in FIGS. 6A-C.

In further embodiments, the backside of the silicon wafer is etchedprior to applying the front side mask.

FIG. 7 is an illustrative mask layout for a photomask 170 that may beused in etching the back side of the wafer to form the target shown inFIGS. 6A through 6F. Circular portion 172 is used to form circular plug158. A cross-shaped aperture 174 is defined by four corner features 176.

Hemispherical Targets

Certain embodiments of the present disclosure provide hemisphericallaser targets. The following discussion provides an example of how suchtargets may be fabricated. Additional construction details, includingparameters for varying the diameter and radius of curvature of thetarget, can be found in Fletcher et al., “Microfabricated Silicon SolidImmersion Lens,” J. Microelectromechanical Sys. 10(3), 450-459(September, 2001), and Strzelecka, “Monolithic Integration of VCSELs andDetector with Refractive Microlenses for Optical Interconnects” (1997)(Ph.D. Dissertation on file with the UC Santa Barbara Library), bothexpressly incorporated by reference herein. As shown in FIG. 8A, asilicon wafer 210 is coated on both front side 206 and the backside 208with a layer of silicon nitride 216. The silicon wafer 210 is spun,exposed, and developed with a coating, such as a 7 μm or 1.6 μm thickcoating, of photoresist 218, such as Shipley 3612 or 220 photoresist. Inparticular examples, the silicon wafer 210 is first primed with HMDS andsoftbaked at 90° C. after the photoresist has been applied.

A suitable backside mask is created and standard photolithographictechniques are used to create a central aperture 220 and two flankingalignment marker apertures 222, shown in FIG. 8B. For example, wafer 210can be exposed to the mask twice for about 3.75 seconds for eachexposure. In some examples, the wafer 210 is developed with LDD26W andthen postbaked at about 110° C. The apertures 220, 222 are created, insome examples, by etching the nitride layer using a RIE process, such asfor about 9 minutes, followed by an STS etch for about 2 hours at anetch rate of about 1.8 μm/minute, such as about 1 μm/minute or less. Inat least certain examples, the entire wafer 210 is exposed, such as forabout 15 minutes, prior to the STS etch. In a particular example, theapertures 220, 222 are etched to a depth of about 420 μm.

With reference now to FIG. 8C, the backside photoresist 218 and anyorganics are stripped, such as using 9:1 H₂SO₄:H₂O₂ for about 20minutes, and the front side 206 of the silicon wafer 210 is spun andexposed with photoresist 226, such as a 10 μm coating of PMGIphotoresist (available from MicroChem, Corp. of Newton, Mass.) or a 7 μmcoating of Shipley 3612 (which may be preceded by a HMDS prime andfollowed by a postbake at 90° C.). In a particular example, the siliconnitride on the front side 206 is first removed, such as by using a RIEetch for about 20 minutes. The front side 206 of the wafer 210 isexposed with a suitable front side mask to create a central target 228,flanking apertures 230, and frame sides 234. For example, the front sidemask may be exposed twice at about 3.6 seconds per exposure. In aparticular example, each target 222 is surrounded by four frame sides226. The frame sides 226 have a relatively large surface area comparedto the target 222 and can be used to protect the target 222.

The front side 206 of the wafer 210 is developed, as illustrated in FIG.8D, and etched, illustrated in FIG. 8E. In a particular example, thefront side 206 is etched using a dry etch process, such as a RIEreactive ion etching (REI) process, to etch approximately 1 μm of thewafer 210. An example of suitable etching conditions is a 20 minute etchat an etch rate of about 3000 Å/minute for silicon and about 300Å/minute for photoresist.

Next, the wafer 210 is treated with an acetone reflow step, creatinghemispherical domes 240 over the target 228 and the frame sides 234,shown in FIG. 8F. The front side 206 is then dry etched, such as with a1:1 Si:photoresist etch ratio until the photoresist has been removed. Ina particular example, an etch of about 20 μm is sufficient to remove thephotoresist. As shown in FIG. 8G, both the frames 234 and the target 228are left with hemispherical domes 244. The target dome 228 is shorterthan the frame domes 234, due to the greater surface area of the frames234. Thus, the target dome 228 is protected by the taller frame domes234. In some examples, the wafer 210 is then hardbaked, such as at about90° C. for about one hour. In a further method, the front side 206 isnot etched following the acetone reflow, and instead the remainingresist is hardened and metal is formed on the domes 240 using anysuitable technique.

Now that the target mold 228 has been formed, one or more metal layers250 can be formed on the front side 206 using any suitable technique,such as E-beam evaporation or sputter coating. An adhesion layer, suchas a Ti adhesion layer, can be used if desired. In a particular example,10 μm of Au is deposited onto the front side 206. In a more particularexample, about 2 μm of Au/Ti is first applied using E-beam evaporationand then an additional 8 μm of Au is applied by sputtering. Theresulting metal coated structure is shown in FIG. 8H.

As shown in FIG. 8I, the silicon underlying the target 222 can then beremoved by etching, such as using a KOH wet etch (such as 33% KOH forabout 90° C. for about 40 minutes), to create a freestanding target 222defining a cavity 256.

FIGS. 9A and 9B illustrate front side 310 and backside 320 masks thatcan be used in the above described process. The front side mask 310includes square frame masks 312, rectangular aligmnent masks 314vertically and horizontally disposed between each of the square framemasks 312, and a central circular target mask 316. The backside mask 320includes rectangular frame masks 324 and a central circular target mask328.

Flat-Top Targets

In some embodiments, it may be useful to provide a relatively largesurface area to which a laser is directed. The surface area can act as aproton or ion source. In addition, the larger surface can be moreuniform than a narrow point, which can allow a generated plasma to bemore fully characterized.

FIG. 10 illustrates a disclosed laser target 400 having a conicalsection 410 and a circular flat top 420. The dimensions of the conicalsection 410 and flat top 420 can be varied as desired. In some examples,the conical section 410 has a diameter of at least about 150 μm. Theconical section 410 has a diameter of about 300 μm in a specificexample. The flat top 420, in particular examples, has dimensions thatare proportional to the base of the conical section 410. The flat top420 can be of various shapes, including square, rectangular, triangular,trapezoidal, parallelogram, pentagon, hexagon, cross, circular,elliptical, or free form or arbitrary shapes and can be hollow or solid.Although section 410 is shown as conical, other shapes can also be used.

The conical section 410 and the flat top 420 can be made of the same ordifferent materials, including metals such as Au, Al, Cu, Mo. The flattop 420, in some examples, is made of a single material. In furtherexamples, the flat top 420 is made of multiple materials. For example,in a specific example, the flat top includes a middle Al layersandwiched by Au layers.

FIGS. 11A-11G illustrate a process for creating a capped target, such asthe target 400 of FIG. 10. As shown in FIG. 11A, standard depositiontechniques, such as thermal oxide deposition or low pressure chemicalvapor deposition (LPCVD) are used to deposit a film of silicon dioxide510 on the front side 514 and backside 516 of a double polished siliconsubstrate 508. In a particular example, at least about 3 μm, such as atleast about 4 μm, of silicon dioxide 510 is deposited. The silicon wafersubstrate 508 can be of any desired crystallography, such as a <100>wafer. In further implementations, the mask material is changed fromsilicon dioxide to other materials, such as nitrides or metals.

As shown in FIG. 11B, standard photolithography techniques are used topattern a desired shape, such as a circle or square, in the backside 516of the wafer 508. In a particular example, 4 μm of Shipley 220photoresist is applied after a HMDS prime. The wafer 508 is thensoftbaked at 90° C., exposed with the mask for about 4.4 seconds,developed with LDD26W developer, and postbaked at about 110° C.

A deep anisotropic etch is performed to create an opening 520approximately halfway into the wafer 508, such as by using a plasma etchat about 3000 Å/minute and then an STS etch at about 2 μm/minute. Thisdeeply etched aperture 520 can help the release of the final target.

Standard photolithography techniques are used, as shown in FIG. 11C, tocreate the desired size and shape of the flat foil, or flat top 420(FIG. 10). For example, the front side 514 can be coated with a layer ofphotoresist 524, developed, and etched to create an opening 526. Forexample, about 12 μm of SPR 220 photoresist can be applied after an HMDSprime, and then the wafer postbaked at 90° C., cured for 4 hours, andthen exposed to the mask for 4 iterations of 3.3 seconds each. The wafer508 can then be developed using LDD26W developer and then postbaked atabout 110° C.

The opening 526 is created, in some examples, using a plasma etch, suchas a 4 μm etch at about 3000 Å/minute. In at least certain examples, theouter mask is proportional to the base of the material that will becomethe conical section 410. The outer mask allows the dimensions of theconical section 410 to be controlled independently from the dimensionsof the flat foil 320.

One or more metals layers 530, such as layers Au or Al, can be depositedinto the flat. The layers 530 may have the same or different shapes,size, or thickness. In a particular example, the flat top is a circularplug of Al. Standard lift off techniques are then used to removeextraneous metal. FIG. 11D illustrates the wafer 508 after the lift offtechnique has been applied and a layer of photoresist 536 has beenapplied and exposed to create apertures 538 flanking the metal layer540.

FIG. 11E shows the wafer after a deep isotropic dry etch has beenperformed, such as by disabling the passivation step of a standard Boschprocess (using little to no CHF₃), such as at about 2 μm/minute. Theetch process is stopped short to avoid completely etching away the endcap and creates cavities 544 flanking a cone structure 548 supporting aflat surface 550 on which the metal layer 540, silicon dioxide layer510, and photoresist layer 536 rest.

The remaining photoresist 536 is removed, such as with acetone or 9:1H₂SO₄:H₂O₂. The remaining mask material 510 is then removed, such aswith a dry etch, such as an O₂ dry etch. As shown in FIG. 11F, anhour-glass mold 548 of silicon is left with a metal top 540. A layer 558of one or more desired target metals, or other target materials, such asAu, is then deposited, such as by sputtering. In a particular example,10 μm of Au is sputtered onto the mold 548. An adhesion layer, such asTi, may be used if desired.

Finally, as illustrated in FIG. 11G, the silicon is etched out of thecone structure 560, such as by using a KOH etch (for example, 33% KOH atabout 100° C. for about 3 to 4 hours) through the opening 520 in thebackside 516 of the wafer 508. In at least certain examples, the wafer508 is over-etched to completely remove silicon from the cone structure560. The wafer 508 can then be KOH decontaminated and stripped oforganics, such as by treatment with 5:1:1 H₂O:H₂O₂:HCl and 9:1H₂SO₄:H₂O₂ for about 20 minutes each at about 70° C. and about 120° C.,respectively.

Although FIG. 10 illustrates a flat foil 420 that encapsulates maskmaterial, other types of top structures can be implemented using thedisclosed techniques. A number of such structures are shown in FIGS.12A-12D. FIG. 12A illustrates a top according the procedure describedwith respect to FIGS. 11A-G having an encapsulated material. The topshown in FIG. 12B includes a hollow top, the mask material having beenremoved, such as through a backside etch process. FIG. 12C illustrates atop formed by etching a portion of the mask material prior toencapsulating the target. This process results in a smaller top since asmaller amount of material is capsulated. This smaller top can behollowed in a manner similar to that for FIG. 12B. FIG. 12D illustratesan embodiment where multiple materials are encapsulated. In someembodiments, the additional material serves as an etch mask. In furtherembodiments, the additional material is left after the etch mask isremoved.

FIG. 13 illustrates a top plan view of a further embodiment of a target600 having a top 610. The top 610 has a horizontal structural connection620 that connects the target 600 to a base piece 630. The base piece 630may aid in handling and positioning the target 600. Holes 640 can beetched into the structural connection 620 in order to minimize changesin the target 600 shape due to the structural connection 620.

The disclosed targets can provide a number of advantageous. For example,the lithographic techniques used to produce the target may allow manytargets to be fabricated and fabricated with consistent properties.Accordingly, the present disclosure may allow targets to be constructedless expensively than using prior techniques. Because of the potentiallylower cost, or greater numbers of targets that can be made, such methodsmay allow the targets to be used in more applications, as well aspotentially increasing the quality or quantity of data available fromtarget experiments. In further implementations, the targets can befabricated with a surrounding support that can help protect the targetfrom damage and aid in handling and positioning the target.

In particular implementations, the disclosed targets can be manufacturedwith a sharp or narrow tip, such as a tip of approximately the samewidth as the wavelength of a laser to be used with the target. Inparticular examples, the width of the target tip is about 1 μm orsmaller. Such tips can result in enhanced energy production. Similarly,the present disclosure can provide targets, and methods of forming suchtargets, having approximately the same size as the spot size of a laserused to shoot the target. Because of the closer size match between thelaser and the target, the target shape may be used to affect the resultsof the target-laser interaction.

Some aspects of the present disclosure provide free standing targets.Free standing targets may produce greater energy and allow for moreaccurate characterization of resulting plasmas if a substrate does notinterfere with the interaction of the laser and target.

Further aspects of the present disclosure provide hemispherical targets.The hemispherical targets can be produced with known lens diameters andradius of curvatures, which can aid in positing the targets and objectswith respect to the target. Control of the fabrication conditions allowsthe target characteristics to be tailored to a particular application.

Capped targets, such as cones capped with a flat top, are provided bysome embodiments of the present disclosure. The cap of such targets canprovide a larger surface for the laser to contact after being guided bythe remainder of the target. The larger surface may be used to producemore energy, or more or different types of radiation. Adjusting thecomposition of the target or cap can allow a desired energy profile tobe obtained from the target. For example, the cap can be created withmultiple metals, which may have the same or different shape, size, orthickness. In some examples, the cap has concentrically arranged metallayers. In further examples, the cap has a layer of metal on whichanother metal is patterned, such as in a polka-dot pattern.

It is to be understood that the above discussion provides a detaileddescription of various embodiments. The above descriptions will enablethose skilled in the art to make many departures from the particularexamples described above to provide apparatuses constructed inaccordance with the present disclosure. The embodiments areillustrative, and not intended to limit the scope of the presentdisclosure. The scope of the present disclosure is rather to bedetermined by the scope of the claims as issued and equivalents thereto.

What is claimed is:
 1. A method for forming a free-standing hollowpointed metal laser focusing target comprising: forming an aperture in afront side of a masked silicon wafer; etching the front side of thesilicon wafer exposed in the aperture until the silicon crystal planesintersect to form a pyramidal void; forming an adhesion layer on thesurface of the pyramidal void; forming a layer of the target material onthe surface of the adhesion layer; etching the back side of the siliconwafer to expose a back surface of the layer of the target material. 2.The method of claim 1 wherein the target material is formed to athickness of at least 1.5 μm.
 3. The method of claim 2 furthercomprising forming a second layer of a target material over the backsurface of the first layer of the target material.
 4. The method ofclaim 3 wherein the second layer of target material is selected from thegroup consisting of Au, Ti, Cu, Mo, Ni, Ta, W, Pd, Pt, and Ag.
 5. Themethod of claim 4 wherein the thickness of the two layers of targetmaterial is greater than 8 μm.
 6. The method of claim 5 wherein theinternal apex of the hollow pointed metal laser focusing target is lessthan 7 μm.
 7. The method of claim 5 wherein the internal apex of thehollow pointed metal laser focusing target is less than 5 μm.
 8. Themethod of claim 5 wherein the internal apex of the hollow pointed metallaser focusing target is less than 1 μm.
 9. A method for forming afree-standing cone or hollow pointed shaped laser focusing targetcomprising: forming an aperture in the front side of a masked siliconwafer, the aperture having a central plug; etching the front side of thesilicon wafer exposed in the aperture until a cone or pointed mold isformed under the plug; forming a layer of a target material on thesurface of the cone or pointed mold; forming an aperture in the backside of the masked silicon wafer that is concentric with the centralplug and is larger than the central plug; and etching the back side ofthe silicon wafer to expose a back surface of the target material. 10.The method of claim 9, wherein forming a layer of a target materialcomprises forming a metal cone or hollow pointed metal structure. 11.The method of claim 10, wherein the target metal is selected from thegroup consisting of Au, Ti, Cu, Mo, Ni, Pd, Pt, and Ag.
 12. The methodof claim 11, wherein the metal cone-shaped or hollow pointed laserfocusing target has an internal apex of less than 7 μm.
 13. The methodof claim 11, wherein the metal cone-shaped or hollow pointed laserfocusing target has an internal apex of less than 5 μm.
 14. The methodof claim 11, wherein the metal cone-shaped or hollow pointed laserfocusing target has an internal apex of less than 1 μm.
 15. The methodof claim 11, wherein the metal cone-shaped or hollow pointed laserfocusing target is formed to a thickness of from 1 μm to 20 μm.
 16. Themethod of claim 11, the metal cone-shaped or hollow pointed laserfocusing target is formed to a thickness of about 8 μm.
 17. The methodof claim 10, wherein the metal is Au and further comprising forming atleast one additional layer of metal on the surface of the Au layerselected from the group consisting of Ti, Cu, Mo, Ni, Pt, Ta, W, Pd, andAg.
 18. The method of claim 10, wherein the metal cone-shaped or hollowpointed laser focusing target has an internal apex of less than 7 μm.19. The method of claim 10, wherein the metal cone-shaped or hollowpointed laser focusing target has an internal apex of less than 5 μm.20. The method of claim 17, wherein the metal cone-shaped or hollowpointed laser focusing target has an internal apex of less than 1 μm.